Tape drive access problem

I asked about this on Ubuntu.org several days ago, but I don't have an answer yet. Let's see if I can get lucky here.

I'm trying to set up an environment that I can use for Drupal development (which I have done in Windows up to now).

To run backups I need to be able to use my tape drive (an IBM Ultrium 1 drive with a SCSI interface). Unbuntu won't let me. It knows the tape drive is there, because there's a file in /dev/tape/by-id which describes the drive precisely. But when I look at my user privileges in “Users and Groups,” the “Use tape drives” item is cleared, and is disabled so that I can't put a tick on it.

I thought I could fix this by opening “Users and Groups” through sudo, but when I did that I found exactly the same thing: “Use tape drives” disabled, can't be ticked.

What's the trick to add the tick?